The present invention relates to a device for gas separation using a membrane adapted to remove contaminants from the feed gas mixture. More specifically it relates to a device in which a feed gas containing contaminants is contacted with an adsorbent material to remove the contaminants within a gas separation membrane module.
Many important industrial gas separation processes utilize the principle that a gaseous mixture may be separated into component gases using a selectively gas membrane which is more permeable to one or more of the components of the mixture than other components. Various membrane configurations are used, such as flat sheet, pleated sheet, spiral wound sheet, ribbon tube membranes and the like.
Common commercial processes carry out the separation using membrane modules in which a selectively gas permeable membrane has the form of an elongated bundle of closely packed, small diameter hollow fibers. U.S. Pat. No. 3,339,341 to Maxwell et al., the complete disclosure of which is hereby incorporated by reference herein, describes typical conventional gas separation membrane modules in detail.
For highly efficient and long-term stable operation, selectively gas permeable membranes normally need the feed gas to be substantially free of contaminants such as heavy hydrocarbons. Contaminants can accumulate on the gas transfer surfaces of the membrane or otherwise interfere with transport of the feed gas components through the membrane. Such interference can over time reduce the flow rate of the gas mixture through the membrane and/or the selectivity of the membrane. If contact of the membrane with contaminants is not prevented, or at least reduced, separation performance can deteriorate rapidly to the extent that module should be replaced.
It is known to use pre-treatment facilities in the form of beds or towers containing adsorbent particles such as activated carbon, to remove hydrocarbons from the feed gas mixture before the feed gas enters the module. While such pre-treatment devices may be effective at removing hydrocarbons, there are a number of disadvantages in using separate pre-treatment devices.
Mainly, conventional pre-treatment facilities take up space. Many common adsorbent particles have very slow adsorption kinetic behavior, i.e., they require long contact between the contaminant-entrained feed gas before significant quantities of the contaminants are removed. Consequently, conventional pre-treatment facilities are designed to have long contact times and this is manifested in very large and bulky equipment. One potential utility for selectively permeable membrane separation systems is the generation of inert gas, i.e., nitrogen, from ambient air on board aircraft for example for the purpose of reducing the explosion hazard of fuel storage tanks. In such an application the extra weight as well as the volume of an additional pre-treatment facility upstream of the module is a major practical disadvantage.
Another drawback of large pre-treatment facilities is that they are expensive to provide, operate and maintain. The beds and towers often contain a large inventory of adsorbent material. After a period of operation, the adsorbent becomes saturated and is replaced or the saturated adsorbent is regenerated. The cost to replace adsorbent material is an obvious expense. Also it can be inconvenient to move fresh and spent adsorbent material to and from the saturated pre-treatment facility. Not infrequently, redundant pre-treatment facilities are provided so that one unit can remain in service while another is serviced or regenerated. The cost of duplicate pre-treatment facilities adds to the complexity and the cost. Additionally, the system needed to regenerate saturated pretreatment facilities often further adds to the complexity of a gas separation operation. In some applications separate pretreatment facilities are not feasible due to considerations of space availability, complexity and/or cost.
There is a need in a gas separation process for a pre-treatment facility for cleaning the feed gas mixture to the membrane which consumes a small amount of space relative to the separation unit. It is also desirable to have a pre-treatment facility that is less expensive and simpler to operate and service than conventional filters upstream of membrane modules.
Accordingly the present invention provides a module comprising a selectively gas permeable membrane for separating components of a gas mixture having gaseous contaminants which can affect the separation performance of the membrane, the module comprising: a shell; a gas separating membrane within the shell; and a filter medium within the shell operative to prevent the contaminants from contacting the membrane.
There is also provided a method of using a membrane to separate components of a gas mixture having gaseous contaminants which can affect the separation performance of the membrane, the method comprising: (1) providing a module comprising a shell; a selectively gas permeable membrane within the shell; and a filter medium operative to selectively remove the contaminants from the gas mixture, the filter medium being positioned between the shell and the membrane; (2) introducing the gas mixture into the shell; (3) conducting the gas mixture through the filter medium, thereby producing a filtered mixture depleted in contaminants relative to the gas mixture; and (4) contacting the membrane with the filtered mixture, thereby forming a permeate gas enriched in more preferentially permeable components of the gas mixture.